Question about semantics of $ variables in json explain plans in 13

[Jerry Brenner]

Is there any documentation on the semantics of $ variables in json explain
plans for both InitPlans and SubPlans in 13?

I'm trying to understand the attached json file.

   - It looks like $0 represents the value from the outer query block when
   the correlated subquery is evaluated
   - It looks like $1 represents the result of the subquery evaluation

Here are the relevant lines from the plan. (I've attached the full plan as
a file.):

                "Node Type": "Subquery Scan",
                "Parent Relationship": "Inner",
                "Parallel Aware": false,
                "Alias": "ANY_subquery",
                "Filter": "(qroot.sendorder = \"ANY_subquery\".col0)",
                "Plans": [
                  {
                    "Node Type": "Result",
                    "Parent Relationship": "Subquery",
                    "Parallel Aware": false,
                    "Plans": [
                      {
                        "Node Type": "Limit",
                        "Parent Relationship": "InitPlan",
                        "Subplan Name": "InitPlan 1 (returns $1)",
                        "Plans": [
                          {
                            "Node Type": "Index Only Scan",
                            "Parent Relationship": "Outer",
                            "Parallel Aware": false,
                            "Scan Direction": "Forward",
                            "Index Name":
"message_u_destinatio_1kk5be278gggc",
                            "Relation Name": "pc_message",
                            "Alias": "qroot0",
                            "Index Cond": "((destinationid = 67) AND
(contactid = $0) AND (sendorder IS NOT NULL))",

Here's a formatted version of the query from the json file:

SELECT /* ISNULL:pc_message.FrozenSetID:, KeyTable:pc_message; */ qRoot.ID
col0, qRoot.CreationTime col1
FROM pc_message qRoot
WHERE qRoot.DestinationID = $1 AND qRoot.Status = $2 AND qRoot.contactID IS
NOT NULL AND qRoot.FrozenSetID IS NULL AND qRoot.SendOrder IN
     (
        SELECT MIN (qRoot0.SendOrder) col0
        FROM pc_message qRoot0
        WHERE qRoot0.DestinationID = $3 AND qRoot0.contactID =
qRoot.contactID)
ORDER BY col1 ASC, col0 ASC LIMIT 100000


Thanks,
Jerry

pc_message415bc88c610f40c448a9c7a3eb19b704_fullqueryplan.json
Description: application/json